In the case of effectively using a flammable gas, it is necessary to separate a gas such as air from a source gas containing the flammable gas, and concentrate the flammable gas to an appropriate concentration range. Various such apparatuses and methods for concentrating a flammable gas have been proposed, and inventions have been proposed in which a gas (so-called coal mine gas) generating from a coal mine in the form of a methane-containing gas is used as a source gas, air (mainly containing nitrogen, oxygen, and carbon dioxide) is separated from the source gas by using an adsorbent, and the methane gas is concentrated for use.
More specifically, Patent Document 1 proposes an invention of an apparatus and a method for concentrating methane gas. According to the invention, with the use of natural zeolite, which has a very slower adsorption rate for methane gas than for nitrogen, as an adsorbent (i.e., with the use of an adsorbent that preferentially adsorbs nitrogen, oxygen, and carbon dioxide to methane gas), coal mine gas is introduced into an adsorption tower filled with the adsorbent by a compressor or the like until a predetermined pressure is reached. Then, nitrogen, oxygen, and carbon dioxide that are contained in the coal mine gas are adsorbed first at the front portion (lower portion) of the adsorbent tower, and methane gas, for which the adsorption rate is slow, is adsorbed at the back portion (upper portion) of the adsorption tower. Further, the methane gas is released from the upper portion of the adsorption tower until it reaches atmospheric pressure.
Thereby, the air can be separated from the coal mine gas as the source gas by using the adsorbent, the methane gas can be concentrated, and the concentrated methane gas can be used as a fuel or the like.
That is, as the PSA cycle, a configuration is conceivable that is provided with a control means for successively performing a flammable gas adsorption step of supplying the source gas to the adsorption tower by a supply and release means, and releasing exhaust gas from the adsorption tower, and a flammable gas desorption step of collecting the flammable gas desorbed by a collecting means.
Further, as such a PSA cycle, it has been considered to perform a pressure equalization step of transferring the gas in the adsorbent tower that has finished the adsorption step and is in the high pressure state into another adsorption tower in a lower pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, and a pressure equalization step of receiving, after finishing the decompression step, the gas from another adsorption tower that is in a higher pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, thereby improving the efficiency of the energy required for pressurizing and depressurizing in the adsorption towers, and increasing the recovery rate of the gas to be concentrated.
Note that in the present invention, of a pair of adsorption towers that perform the pressure equalization steps, the pressure equalization step performed by the adsorption tower whose internal pressure is lowered by transferring the gas to the other tower is referred to as the pressure equalization (depressurizing) step, and the pressure equalization step performed by the adsorption tower whose internal pressure is increased by receiving the gas from the other tower is referred to as the pressure equalization (pressurizing) step.
In addition, Patent Document 2 further discloses a helium gas concentration method including:
providing four adsorption towers each filled with an adsorbent that adsorbs methane gas and nitrogen; and
in the case of performing, for each of the adsorption towers, a PSA cycle of performing:
an adsorption step of receiving supply of a helium-containing methane gas, adsorbing methane gas on the adsorbent, and recovering helium gas;
a pressure equalization (depressurizing) step of transferring the gas in the adsorption tower that has finished the adsorption step and is in the high pressure state to another adsorption tower in a lower pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state;
a decompression step of, after the internal pressure of the tower has been lowered by the pressure equalization (depressurizing) step, further decompressing the adsorbent to a low pressure state so as to desorb the methane gas adsorbed on the adsorbent;
a pressure equalization (pressurizing) step of receiving, after finishing the decompression step, the gas from another adsorption tower that is in a higher pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state; and
a pressurizing step of, after the internal pressure of the tower has been increased by the pressure equalization (pressurizing) step, further supplying a pressurizing gas into the adsorption tower so as to restore the adsorbent into a high pressure state capable of selectively adsorbing the methane gas,
two different pressure states of the internal pressure of the adsorption tower are set as the intermediate pressure state,
the method includes, as the pressure equalization (depressurizing) step,
an initial pressure equalization (depressurizing) step of transferring the gas in one of the adsorption towers that is in the high pressure state to another one of the adsorption towers that is in an intermediate pressure state at a pressure lower than the pressure of the one of the adsorption towers so as to bring the pressure in the one of the adsorption towers into a high pressure-side intermediate pressure state, and
a final pressure equalization (depressurizing) step of transferring the gas in one of the adsorption towers that is in the high pressure-side intermediate pressure state at a pressure higher than the low pressure state to another one of the adsorption towers that is in the low pressure state so as to bring the pressure in the one of the adsorption towers into a low pressure-side intermediate pressure state, and
the pressure equalization (pressurizing) step includes:
an initial pressure equalization (pressurizing) step of receiving the gas in one of the adsorption towers that is in the high pressure-side intermediate pressure state into another one of the adsorption towers that is in the low pressure state so as to bring the pressure in the one of the adsorption towers into the low pressure-side intermediate pressure state; and
a final pressure equalization (pressurizing) step of receiving, into one of the adsorption towers that is in the low pressure-side intermediate pressure state, the gas in another one of the adsorption towers that is in the high pressure state so as to bring the pressure in the one of the adsorption towers into the high pressure-side intermediate pressure state.